Abstract:
Outputs from a Water Balance and Transport Model (WBM/WTM) were used to determine the spatial distribution of renewable water supply, expressed as the sum of local runoff and river corridor discharge. Monthly atmospheric forcings from 1960-95 were from (New et al., 1998). Estimates of domestic and industrial water demands (Vorosmarty et al., 2000a; 2004) were apportioned by urban/rural population ... densities. Agricultural withdrawals were based on African water statistics (Jippe Hoogeveen, FAO/AGL, Rome Italy) at the sub-basin level, and a mapping of irrigation-equipped lands (Siebert et al., 2002). All supply and demand estimates were resampled as required and georegistered to a 6 min grid and river network (STN-06), updated from a previous flow topology (Vorosmarty et al., 2000b) using a network rescaling algorithm that processed 1-km digital streamlines (Fekete et al., 2001). STN-06 basin boundaries were compared to a hand-corrected database provided by FAO. Water scarcity was evaluated, in part, by computing the Climatic Moisture Index (CMI, Willmott and Feddema, 1992), the ratio of annual precipitation (P) to annual potential evapotranspiration, (PET). Specifically CMI = (P / PET) -1 when P < PET and CMI = 1 - (PET/P) when P = PET. The CMI ranges from -1 to +1 , with wet climates showing positive values, dry climates negative. PET was estimated using a physically based function (Shuttleworth and Wallace, 1985). We grouped CMI into major climate categories following Koppen. The coefficient of variation (CV) computed for all variables is the ratio of the standard deviation to the mean over the time series analyzed.

Water supply in each grid cell (n) has two sources: locally-generated discharge (QLn) and river corridor discharge (QCn), which enters from upstream cells. QLn is the product of runoff (Rn) and cell area (An). QCn accumulates QLn in a downstream direction along the STN-06 digital nertwork. Cells with mean upstream runoff < 3 mm yr-1 were considered inactive or non-perennially discharging (Vorosmarty et al., 2000b). Water use is represented by local demand (DIAn), the sum of domestic, industrial and agricultural water withdrawals. Dividing DIAn by QCn yields an index of local relative water use. A high degree of stress is indicated when the relative water use index is > 0.4 or 40% (34). DIAn summed in a downstream direction (in a similar manner as QCn) and divided by QCn is called the water reuse index and represents the extent to which runoff is recycled or reused as it accumulates and flows toward the basin mouth. The water reuse index typically increases in a downstream direction, indicating reuse and recycling of river corridor water. This index can, however, decrease when mainstream flow is diluted by more pristine (less-recycled) tributary waters.

- Annual CMI, coefficient of variabiltiy (CV): Interannual variability of the CMI, as represented by the CV, which is computed as the standard deviation divided by the mean: Willmott and Feddema, 1992.

- Annual discharge (Q), annual coefficient of variability (CV). Interannual variability of discharge, as represented by the CV, which is computed as the standard deviation divided by the mean: Vorosmarty et al., 2005.

- Months with RWSI exceeding 0.4. Number of months within an average years during which the RWSI equals or exceeds 0.4: Vorosmarty et al., 2005.

- Number of people exposed to water stress: Number of people (per grid cell) within grid cells that experience RWSI equal to or greater than 0.4, on an average annual basis: Vorosmarty et al., 2005.

- Number of people exposed to water stress: Number of people (per grid cell) within grid cells that experience RWSI equal to or greater than 0.4, for the 30 year minimum discharge: Vorosmarty et al., 2005.

- Water reuse index (WRI), mean annual. Water reuse index is computed as the ratio of cumulative DIA to mean annual discharge, representing the degree to which river water is reused by humans as it flows along a river network: Vorosmarty et al., 2005.